Squirrel cage rotors comprise circumferentially spaced rotor bars secured within a laminated iron core which are electrically and mechanically connected to end rings.
Depending on the mechanical and electrical properties desired, rotor bars may be made of various copper alloys and heat treated at different temperatures. The temperature at which the rotor bars and end rings are heat treated is known as the "aging or aged temperature". Some rotor bars and end rings for squirrel cage rotors have been made of a chrome copper alloy which has been heat treated to temperatures in the range of 900 degrees to establish the desired mechanical and electrical properties of the rotor bars and end rings. Other rotor bars and end rings have been made of copper, copper alloys or other materials, which are work-hardened to establish the mechanical and electrical properties. In some cases, a combination of work hardening and heat treatment is used. In any event, heating rotor bars and end rings beyond a certain temperature will change their properties. The temperature at which those properties deleteriously change, regardless of the method by which they are obtained (heating, work hardening or otherwise), is referred to herein as the "overaging or overaged temperature".
Brazing is one process by which rotor bars may be joined to the end rings of the rotor. Brazing is defined by the American Welding Society as
"A group of welding processes which produces coalescence of material by heating them in the presence of a filler metal having a liquidus above 840.degree. F. (450.degree. C.) and below the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action."
The braze filler metal determines the temperature required for brazing.
The melting temperatures of commonly used braze filler metals used to braze rotor bars to end rings range from 1150.degree. F. to 1500.degree. F. During brazing, it is necessary to bring the portion of the end ring and rotor bars to be brazed to the melting temperature of the braze filler, which is significantly above the aging temperature of the rotor bars and end rings. Brazing thus subjects the rotor bars and end rings to temperatures that exceed their aging temperature; hence, they are overaged. When the rotor bars or end rings are subject to such temperatures, their mechanical properties are degraded in proportion to the temperature and the time which the rotor bars or end rings are exposed to the elevated temperature. To compound the problem, the rotor core, rotor bars and end rings act as a large heat sink. Thus, heat or energy applied to the joint during brazing is drawn away almost as fast as it is applied if brazing is attempted when the rotor bars and end rings are at room temperature.
U.S. Pat. No. 4,249,098 states that when rotor bars are soldered to end rings, the rotor bars would suffer a loss in strength by soft annealing, and the bottom stacks of the laminated iron core and the insulating layers between them will be subjected to unduly high temperature. That patent discloses a soldering operation in which the end ring is heated to the melting temperature of the solder, causing the solder to melt and fill the gaps between the end faces of the rotor bars and the adjoining surface portions of the end rings.